This application relates to solid state radiation imagers and in particular to a common passivation barrier for transistor and photodiode components and the method of fabricating such an imager.
Solid state imagers typically include a photosensor array coupled to a scintillating medium. Radiation absorbed in the scintillator generates optical photons which in turn pass into a photosensor, such as a photodiode, in which the optical photon is absorbed and an electrical signal corresponding to the incident optical photon flux is generated. Substantially hydrogenated amorphous silicon (a-Si) is commonly used in the fabrication of photosensors due to the advantageous photoelectric characteristics of a-Si and the relative ease of fabricating such devices. In particular, photosensitive elements, such as photodiodes, can be formed in connection with necessary control or switching elements, such as thin film transistors (TFT's), in a relatively large array.
Imager and display arrays are typically fabricated on large substrates on which many components, including TFTs, address lines, and devices such as photosensors, am formed through the deposition and patterning of layers of conductive, semiconductive, and insulative materials. The array is comprised of pixels, with the address lines and associated TFTs being coupled together to enable the photosensor in each pixel of the array to be respectively addressed, so that, for example, the charge developed by each photosensor can be selectively read. The TFT fabrication process involves several patterning steps to produce the desired arrangement of a channel region between a source and a drain electrode with the desired arrangement of semiconductive material disposed between the electrodes and over the gate electrode. The TFT is electrically coupled to a respective photosensor, such as a photodiode, which is disposed to absorb incident photons and accumulate the resulting charge produced in the diode.
The fabrication process for such an imaging array typically includes steps to fabricate the TFT, including forming source and drain electrodes and associated contact pads to address lines and the photosensor. The TFT and related assemblies are then coated with a protective passivation coating, which must then be patterned to provide via openings for electrical contact between the photosensor body and an underlying photosensor electrode (or contact pad, which couples the photosensor to the TFT. The a-Si for the photodiode body is then deposited and patterned; the presence of the protective passivation layer over the TFT assemblies is necessary to ensure that a-Si portions of the TFT are not damaged by the patterning process for the photosensor island (as the etchants used to form the a-Si body would similarly etch TFT components if allowed to come in contact with such a-Si portions).
As the conventional fabrication process involves the deposition of a separate passivation layer over the TFT before fabrication of the photosensor body, a portion of the passivation material abuts the photosensor contact pad in the form of a "lip" or build up of the TFT protective passivation layer material over the edges of the photosensor island bottom contact pad. This buildup of passivation material results in the subsequently deposited a-Si conforming to the shape of this lip, resulting in the lip portion of the passivation layer being disposed between the overlying a-Si material and the photosensor island bottom contact pad. It has been observed, however, that such an arrangement results in increased photodiode leakage, especially when the thickness of the passivation material between the a-Si of the photosensor island and the bottom contact pad is thicker than about 0.5 .mu.m.
In the conventional array formation process, after formation of the photosensor island another passivation layer is deposited over the array to provide electrical insulation between the photosensor island (except at a predetermined contact area) and the common electrode of the photosensor array, which is disposed over the passivation layer. Further, the passivation layer over the photosensor protects the a-Si photosensor from environmental conditions (such as moisture) that can degrade its performance and also protects the array from exposure to materials, such as solvents, used in remaining steps of the fabrication process. Typically a scintillator is then deposited over the common electrode of the photosensor array to complete the imager wafer structure.
An object of this invention is to provide a method of fabricating an imager array having fewer steps and resulting in the elimination of one dielectric layer in the assembled array.
It is a further object of this invention to provide an imager having a structure with a single common passivation layer for the TFT and photosensor in each pixel of the array.
A still further object of the invention is to provide an imager having a photodiode structure conducive to low leakage operation of the photodiode.